Many high end microwave assemblies are still hermetically packaged, such as transmit and receive modules for phased arrays, components for defense applications, power amplifiers, assemblies requiring chip and wire construction, high performance devices and circuit operating in the upper microwave and mm-wave bands, and so on. The reasons include ensuring reliability under environmental variations and a lack of organic or polymer protective layers that do not reduce or interfere with device performance due to factors such as dielectric loss or attenuation, or due to changing the electrical impedance of transmission lines contained in or on the device. Hermetic packaging has substantial drawbacks. The substantial cost and yield impact may be attributed to the specialized nature of the parts used in hermetic packaging, such as metal or ceramic housings, solderable or weldable lids, hermetic seals such as glass-metal seals for connectors, and the manual labor usually required for assembly and test and rework. Meanwhile, most consumer electronics traditionally operating at lower frequencies have been able to move to lower cost non-hermetic packaging through the use of protective coatings, underfill polymers, encapsulants and the like. Such approaches enable more automated batch production on large area circuit boards. Non-hermetic cavity packaging has been done in some cases; however, in environments where there is high humidity and fluctuating temperatures, condensation of water can occur inside the package. In addition, ionic contaminates such as sodium, potassium, and calcium can come from environmental sources including fingerprints, the air or salt water can penetrate many non-filled cavity structures and produce electrical reliability problems such as conductivity between circuits and/or corrosion. The problems from such condensation and ionic contaminates can be eliminated by employing polymer or silicone encapsulations over the electronics components.
Traditional approaches used to package devices for low frequency electronics do not work well on microwave devices and the circuit boards on which they are mounted because of the field interaction from the transmission lines in the circuits extend into the surrounding mediums and often extend into the encapsulants or coatings producing problems such as attenuation, changing transmission line impedance, and otherwise interfering with the function of the circuit.
A possible solution to this problem, as further disclosed herein, is the use of a very low-k layer of material, such as a syntactic foam, that does not substantially interfere with the operation of a circuit designed for operation in air or in a vacuum environment. Such a layer can be applied thick enough to minimize field interactions, for example 0.5 to 2 mm or more thick, and can be used as a “spacer layer” to an outer protective set of layers. While syntactic foams containing solid glass micro-bubbles may have the advantage of low dielectric constants, for shielding from electromagnetic interference and other phenomena, and can be made to isolate microwave devices from ionic contaminants, known materials also exhibit mismatches in the coefficients of thermal expansion among the encapsulants, substrates and electronic/microwave devices. Such mismatches can be detrimental to overall device function over the life of the device. Disclosed and claimed herein are materials that ameliorate the deficiencies of known materials.